Battery charging systems for cell phones and other portable electronics have become commonplace. In normal operation, a charger supplies current to a device (i.e., cell phone) to charge its battery. As the cell phone battery nears full charge, the load current or current draw by the device reduces, so it is advantageous to shut off the charger to reducing energy usage. Additionally, it is advantageous to shut off the charger when device is unplugged from the charger so as to allow the charger to remain in a very low power state while plugged into a wall receptacle, greatly reducing standby power consumption, commonly referred to as “vampire” power.
Turning to FIG. 1, an example of a conventional integrated circuit (IC) 100 having a load switch 106 can be seen. This load switch 106 is generally used to switch and control power from a power supply 108 (i.e., charger) to load 110 (i.e., battery). Load switch 106 generally comprises transistors Q1 and Q2 (which can, for example be NMOS transistors, as shown or PMOS transistors) that are arranged in a back-to-back configuration to perform reverse current blocking by the body diode connections. The IC 100 also generally includes a comparator 102 and control logic 104 to control the operation of the load switch 106. In operation, the power supply 108 is coupled to the input pin VIN so as to power circuitry internal to the IC 100 (i.e., comparator 102 and control logic 104) in conjunction with the ground pin GND and provides power to the load switch 106 through the sense resistor RSEN and sense pin SENSE. The input terminals of the comparator 102 are coupled to the input pin VIN and sense pin SENSE so as to sense the current traversing the load switch 106 to the output pin OUT and load 110, when active. Typically, this sense resistor RSEN has a low resistance (i.e., about 10 mΩ to about 20 mΩ) so as to maintain a low power loss through the resistor RSEN. As a result of monitoring the differential voltage across the sense resistor RSEN, the comparator 102 is able to generate control signals that correspond to a light load condition, a normal load condition, or an overload condition. Based at least in part on the control signals from comparator 102, the control logic 104 can control the load switch 106 (i.e., activate or deactivate).
There are some drawbacks to the IC 100; namely, detection of a light load condition can be problematic. Usually, for light load conditions, the differential voltage across the sense resistor RSEN can be between 0-10 mV, but, at this range, comparators (i.e., 102) are not generally very accurate. Therefore there is a need for improved controls for a load switch.
Some other conventional circuits are: U.S. Pat. No. 6,812,715; U.S. Pat. No. 7,859,132; and U.S. Patent Pre-Grant Publ. No. 2010/0090755.